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Skipjack
General
Designers	NSA
First published	1998 (declassifed)
Cipher detail
Key sizes	80 bits
Block sizes	64 bits
Structure	unbalanced Feistel network
Rounds	32
Best public cryptanalysis
31 rounds are susceptible to impossible differential cryptanalysis.

In cryptography, Skipjack is a block cipher — an algorithm for encryption — developed by the U.S. National Security Agency (NSA). Initially classified, it was originally intended for use in the controversial Clipper chip. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher designs of a government intelligence agency.

Contents 1 History of Skipjack 2 Description 3 Cryptanalysis 4 In pop culture 5 See also 6 References 7 External links

Skipjack was proposed as the encryption algorithm in a US government-sponsored scheme of key escrow, and the cipher was provided for use in the Clipper chip, implemented in tamperproof hardware. Skipjack is used only for encryption, the key escrow is achieved through the use of a separate mechanism known as the Law Enforcement Access Field (LEAF).

The design was initially secret, and was regarded with considerable suspicion by many in the public cryptography community for that reason. It was declassified on 24 June 1998.

To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm (Brickell et al., 1993). The researchers found no problems with either the algorithm itself or the evaluation process. Moreover, their report gave some insight into the (classified) history and development of Skipjack:

[Skipjack] is representative of a family of encryption algorithms developed in 1980 as part of the NSA suite of "Type I" algorithms... SKIPJACK was designed using building blocks and techniques that date back more than forty years. Many of the techniques are related to work that was evaluated by some of the world's most accomplished and famous experts in combinatorics and abstract algebra. SKIPJACK's more immediate heritage dates to around 1980, and its initial design to 1987...The specific structures included in SKIPJACK have a long evaluation history, and the cryptographic properties of those structures had many prior years of intense study before the formal process began in 1987. — SKIPJACK Review, Interim Report, 1993.

Skipjack uses an 80-bit key to encrypt or decrypt 64-bit data blocks. It is an unbalanced Feistel network with 32 rounds. It was specially designed to replace DES.

Eli Biham and Adi Shamir discovered an attack against 16 of the 32 rounds within one day of declassification, and (with Alex Biryukov) extended this to 31 of the 32 rounds within months using impossible differential cryptanalysis.

Truncated differentials and later a complementation slide attack was published against all 32 rounds of Skipjack cipher. It was found, however, that the attacks are flawed. Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.

An algorithm named Skipjack forms part of the back-story to Dan Brown's 1998 novel Digital Fortress. In Brown's novel, Skipjack is proposed as the new "public-key encryption standard", along with a back door secretly inserted by the NSA ("a few lines of cunning programming") which would have allowed them to decrypt Skipjack using a secret password and thereby "read the world's email". However, when Skipjack is released for public peer review, a programmer discovers and announces the existence of the back door, effectively ending the chances of the standard being adopted.

Additionally, in the Half-Life 2 modification Dystopia, the "encryption" program used in cyberspace apparently uses both Skipjack and Blowfish algorithms.

• Initial observations on Skipjack (Biham et al.)
• Specification of Skipjack (PDF)
• SCAN's entry for the cipher
• Bruce Schneier's comments on declassification
• fip185 Escrowed Encryption Standard EES

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